1. Field of the Invention
The present invention relates to a capacitive acceleration sensor in which the moving part for detecting acceleration changes is disposed in a hermetically sealed air gap.
2. Background Art
Capacitive acceleration sensors measure acceleration by detecting a change in the capacitance of a capacitor. This capacitor includes a moving part (sometimes referred to as a “mass body”) and a stationary part. In the typical construction of a capacitive acceleration sensor, the moving part is supported by a beam so that the part is slightly displaced by acceleration and the resulting change in the capacitance of the capacitor is used to detect the acceleration.
This moving part is hermetically enclosed in an air gap formed by a sealing structure so that the part is movable through a certain range of movement. Because of its being hermetically enclosed, the moving part is protected from foreign objects and moisture. The sealing structure is often made up of an upper glass and a lower glass between which is vertically sandwiched the silicon including the moving part and the stationary part. The sealing structure is desired to have an air gap for accommodating the moving part and to prevent the moving part from being affected by foreign objects and moisture.
It is common that the sealing structure is sealed with a resin into a package by transfer molding in order to achieve smaller size, lower cost, and higher performance. In transfer molding, a molding resin is injected at high pressure by the injection molding machine. Therefore, when a capacitive acceleration sensor is subjected to injection molding, a high pressure is applied to the sensor. This high pressure has been found to break the sealing structure at the portion forming the air gap. Such breaking of the sealing structure has resulted in breakage of the moving part and stationary part for detecting acceleration and also resulted in a foreign object entering into the air gap, causing a variation in the characteristics of the acceleration sensor.
In order to avoid breaking of the sealing structure, the injection molding pressure may be reduced. It has been found, however, that such reduction in the injection molding pressure slows down the flow of molding resin and causes formation of voids in the molding resin, thus degrading the internal portion-protecting capability and hence reliability of the sealing structure. Degradation of the reliability has also resulted from detachment of the capacitive acceleration sensor from the molding resin due to reduced adhesion between them.
Another way to avoid breaking of the sealing structure may be to dispose a support pillar in the air gap formed by the sealing structure. Specifically, the support pillar may be disposed so that the top and bottom of the air gap are respectively supported on the opposite ends of the support pillar in order to prevent breaking of the sealing structure. It has been found, however, that if the support pillar is set in an electrically floating state, the moving part is displaced by the electrical action of the support pillar, thereby lowering the measuring accuracy of the acceleration sensor.